The present invention relates to a solenoid valve for controlling a fuel injector.
Solenoid valves are used to control fuel injectors in a fuel injection system having a valve needle, the open and closed positions of which may be controlled by the solenoid valve.
The solenoid valve has a valve ball, which lifts up and opens a valve seat when current flows through the magnet assembly of the solenoid valve. This valve seat is in hydraulic connection with the control pressure chamber of the fuel injector via a borehole. When the valve seat opens, the pressure in the pressure chamber of the fuel injector drops, and the fluid (pressure medium) flows through the borehole in the direction of the valve seat and further into a pressure relief chamber. This causes the valve needle or the fuel injector to open.
It is believed that the common rail injector (CRI) operates according to this conventional operating principle, which permits a main injection and a pilot injection having very brief injection times. Such a solenoid valve is referred to, for example, in German Published Patent Application No. 196 50 865.
Cavitation may cause severe damage to the valve seat of the valve part. The borehole extending through the valve part includes a cylindrical A-throttle adjoining a pilot borehole in the control pressure chamber of the fuel injector, and a subsequent cylindrical diffuser bore leading to the valve seat. The cavitation damage may, for example, occur in the region of an abrupt transition from the diffuser bore to the valve seat. This damage may cause xe2x80x9cwashoutxe2x80x9d of the seat edge. As the damage increases, this edge may break off, resulting in total failure of the injector and operational failure of the vehicle. To solve this problem, the formation of cavitation bubbles should be reduced, and the site of implosion of any remaining bubbles should be shifted to a location, such that this effect no longer influences the correct functioning of the injector.
An exemplary solenoid valve according to the present invention includes a borehole which has, at least in part, one or more sections having a cross section that continuously expands in the direction of the valve seat. Sharp-edged transitions within the borehole, for example, in the transition region from the A-throttle to the diffuser bore, may thus be avoided. It is believed that a conical geometry of the expanding section is advantageous.
A severe separation in flow may occur when the fluid (pressure medium) flows through the A-throttle to the outlet edge downstream, which is sharp-edged due to the manufacturing process, toward the diffuser bore. Dead water and recirculation areas may form at those locations. These effects may result in fluctuations in the reproducibility of the amount of fluid flowing through, as well as in the formation of zones at partial vacuum and cavitation bubbles.
Further within the borehole, the flow again contacts the bore walls. Shortly before reaching the throttle point at the valve seat situated further downstream, the pressure in the medium rises again and the cavitation bubbles floating in the liquid stream implode, thereby causing the described cavitation damage at the wall of the flow channel.
As a result of the borehole of an exemplary solenoid valve according to the present invention, the flow geometry in the valve part is altered, so that a generally turbulence-free transition of the medium from the A-throttle to the valve seat may be achieved without the described negative effects.
The transition from the A-throttle to the diffuser bore may, for example, be formed with a continuously expanding cross section, so that the borehole includes three sections that merge into one another. In this manner, separation of the flow at the sharp-edged outlet edge may be prevented.
Furthermore, the borehole, for example, may be divided into three sections: the A-throttle, the diffuser bore adjoining the section expanding in cross section, and the diffuser bore, the A-throttle and the diffuser bore having substantially the same length. It is believed that, in conventional designs, the A-throttle directly adjoins the diffuser bore, the latter having a greater length than the former. In an exemplary embodiment according to the present invention, both the A-throttle and the diffuser bore may be considerably shortened, thereby lowering the pressure, for example, in the diffuser bore. In conjunction with the continuously expanding (e.g., conical) transition region between the A-throttle and the diffuser bore, an optimum shape of the flow channel may be obtained, in which no cavitation bubbles are formed, and no implosions of these bubbles are observed.
In another exemplary embodiment according to the present invention, the borehole upstream from the valve seat has multiple, for example, conical, sections expanding in the direction of the valve seat. A good flow pattern may be obtained when each of the two cylindrical boreholes, e.g., the A-throttle and the diffuser bore, has a conically shaped section. For example, the length of the (cylindrical) diffuser bore may be reduced, so that the pressure rise within the diffuser bore is no longer sufficient to allow the implosion of any cavitation bubbles that may form. As described above, the conical sections connecting the cylindrical boreholes prevent separation of flow and, thus, prevent the cause of cavitation bubble formation.
The aperture angles of the successive conical sections in the direction of the valve seat may, for example, increase, thus permitting a gradual transition to the aperture angle of the valve seat. This may create an favorable flow pattern.
The sections that continuously expand in cross section, for example, may be created in a simple mechanical fashion by rounding off the respective transitions between the boreholes, such as the A-throttle and the diffuser bore. In this manner, the sharp edge of a transition may be machined during manufacturing to provide an optimum flow channel.